Slew control is generally important in various circuits, such as LED output stages. In many applications, though, programmability of the rise and fall times is missing, which may not allow many circuits to meet desired specification. The timing diagram for such a current slew control circuit can be seen in FIG. 1. As shown, there is a particular ramp time for the gate voltage of a switching transistor (from about 3.3V to about 0V). Part of this ramp time is used for current slewing while the other part (between the rising edge of the enable signal EN and the trigger voltage level to activate the current mirror) is the “turn-on” delay of the current mirror. The problem is propagation delay time is very closely related to the rise/fall time. If the parameters are designed to achieve desired proper rise/fall time, propagation delay can be significantly long and cannot meet the performance requirements.
Some examples of conventional circuits are: U.S. Pat. Nos. 7,250,810; 7,298,173; and 7,474,141.